simd/archsimd: reword documentation of shfit operations

simd/archsimd: reword documentation of shfit operations

Say the name of the parameter, instead of "the immediate".

Don't say "Emptied bits are zeroed", which is implied by the shift
operation in Go (and perhaps many other languages). For right
shifts, say signed or unsigned shifts instead.

Change-Id: I29c9c0e218bfaeef55b03d92d44762e34f006654
Reviewed-on: https://go-review.googlesource.com/c/go/+/730720
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>